1. Field of the Invention
The present invention relates to a technical field of a digital signal delay measuring circuit for measuring a delay time of a digital signal in a digital circuit.
2. Description of the Related Art
In recent years, a digital circuit inside an IC (Integrated Circuit) has an outstanding tendency toward an operation speed-up and a large circuit scale due to fine patterning. However, the speed-up may cause a severer problem of fluctuation in delay time resulting from manufacturing process, which becomes a serious problem in IC mass production. Therefore, in the manufacturing test of a high-speed operating IC, delay fault diagnosis that is one scan test is carried out to determine a defective product.
As one of the delay fault diagnosis methods, there is often performed a real-time scan test (At-Speed Test). This method is such that a shift operation of scanned data is performed with a low-speed scan clock that is used in a usual scan test and an actual circuit operation test is performed with a user clock inside the IC.
FIG. 1 shows the real-time scan test. The two cycle clocks, LaunchCLK and CaptureCLK, in FIG. 1 are the user clock used actually, and generally, they show higher speed than that of the shift clock output from an IC tester for example. The delay fault diagnosis is performed with the use of these LaunchCLK and CaptureCLK by operating the circuit with the actually-used user clock.
Meanwhile, as the IC manufacturing test needs to take shorter time, more circuits than those used practically are to operate simultaneously. As a result, more current than that of practical use is made to pass through the IC, and there occurs a large IR-Drop (supply voltage drop).
As the IR-Drop increases a delay time of each element, the delay fault diagnosis sometimes results in error. The IR-Drop is broadly divided into a static IR-Drop and a dynamic IR-Drop. The static IR-Drop is a phenomenon where the supply voltage drops even when the circuit is out of operation. On the other hand, the dynamic IR-Drop is a phenomenon where the supply voltage drops by the circuit operating actually. In the former, measurement of the delay time is easy because the delay time can be measured while the IC operation is stopped. However, in the latter, measurement per se is difficult. Particularly in the case of a large-scale synchronous circuit, many circuits operate simultaneously with change in clock, and therefore, the dynamic IR-Drop in the large-scale synchronous circuit becomes a voltage drop phenomenon occurring in an extremely short time with clock change.
FIG. 2 shows the dynamic IR-Drop image. As shown in FIG. 2, the dynamic IR-Drop is a supply voltage drop phenomenon that occurs in an extremely short time. This phenomenon depends on the number of circuits that are under operation. It also depends on the current supply capacity of the supply circuit. Hence, the dynamic IR-Drop varies depending on the physical position inside the IC. Further, the dynamic IR-Drop depends on the operation states of circuits (activation rate of the circuits) even when they are at the same position. Therefore, measurement is difficult for the dynamic IR-Drop.
A synchronous circuit operates depending on clock change. Therefore, it is greatly susceptible to the dynamic IR-Drop and results in larger delay time. Specifically, for recent large-scale synchronous circuits, simultaneously-operating circuits are increased in scale with increase in influence of the dynamic IR-Drop, which causes a bottleneck for the delay fault diagnosis. In other words, when the influence on the circuit delay by the dynamic IR-Drop is large, the delay fault diagnosis highly probably results in error. Specifically, in the scan test of the large-scale synchronous circuits, the scale of circuits operating simultaneously becomes larger. This tends to enlarge the delay due to the dynamic IR-Drop and to increase the possibility of diagnosis error.
Conventionally, as the method for measuring an increase of delay time due to IR-Drop, there has been widely performed measurement with a ring oscillator disclosed, for example, in Patent Document No. 1. Patent Document No. 1: Japanese Patent Application Laid-Open No. 2004-146612